428 Climate of the Glacial Period. (October, 
in that way the mean temperature would be greatly 
lowered. 
Mr. Croll puts his theory briefly in these words :—‘‘ The 
southern hemisphere is further from the sun during its winter 
than the northern, and therefore cools more rapidly. It is, 
however, nearer to the sun during its summer than the 
northern, and on this account cools more slowly. The heat 
thus saved during summer would exactly compensate for 
that lost during winter were the two periods of equal length ; 
but as the southern winter is longer than the southern 
summer by more than 74 days, there is on the whole a 
greater amount of heat lost during winter than is saved 
during summer.” ‘‘ The greater length of the winter half 
year over the summer half, when the eccentricity is near its 
maximum, would affect the climate in two different ways :— 
(1), by allowing the ground to cool by radiation to a greater 
extent than it would otherwise do were the (summer) 
seasons of greater length; and (2), by lengthening the ice- 
accumulating period and shortening the ice-melting period. 
The influence of the first cause upon the glaciation of the 
country would probably be felt to a considerable extent ; but 
it is to the second that we must attribute the principal 
effect.”* The above was written in 1865, but I cannot find 
that Mr. Croll has modified his theory in any later writings ; 
and Mr. James Geikie, his colleague on the Geological 
Survey of Scotland, has, during the present year, in his work 
“‘The Great Ice Age,” adopted it, and in discussing it has 
described it substantially as above. Now if it be true that 
the hemisphere, that has its winter when the earth is farthest 
from the sun, will have its mean temperature reduced by 
an excess of radiation; whilst that of the opposite hemi- 
sphere will be correspondingly increased, we have certainly 
a true cause of former great oscillations of climate. Before, 
therefore, entering on the consideration of some other 
causes that would, according to Mr. Croll, be brought into 
action and intensify the effects, it will be well to examine 
the fundamental bases of the theory. 
Ist. Would there be more radiation of heat into space, 
and consequently increased cold, at times of the greatest 
ellipticity of the orbit in that hemisphere whose winter 
happened when the earth was furthest from the sun? 
Mr. Croll, as we have seen, answers in the affirmative, and 
in the shape in which he puts it it appears as if it would be 
so. As at that time the number of hours of night in each 
* Reader, 1865, p. 631. 
